Can somebody explain this?

If I turn the unit on before I take the bike outside, it will often claim to have found the GPS signal and I can set off immediately.

If I turn it on after I leave the house, I have to wait a minute or two for it to get a signal before I can set off.

However, if I have turned the unit on before I took the bike outside, it will initially give completely random speed readouts, often claiming I am doing 20mph when I am doing walking speed. Eventually it will stabilize, but it never seems to fully sort itself out.

If I wait until I get outside before I turn it on, and then wait for the GPS signal, it works OK.

Why does this happen? I thought GPS used the satellites to determine your speed etc, so why can it not figure it out if I turned the unit on indoors before I set off?

Might I have a defective unit, or am I not understanding properly how GPS works?

First thing, the closer you are to the last location where it was turned on (in your house, that would be when you connected it to upload data), the faster it is able to figure out where it is, because it knows what satellites to start looking for. In your house, that signal will be fairly inaccurate at times, and will give you those erroneous speed readings.

When you've moved from the last used location or it has been a long time since you turned it on (usually weeks or longer), its information about which satellites to look for will be out of date, and it will have to download new data (called ephemeris) from one of the few satellites that provides this information. This can take awhile.

I have tried firing up a GPS in iffy conditions (valley, deep canopy) after a long plane ride and had it take hours to get a signal lock because it wasn't able to pick up one of the few satellites providing ephemeris and it had to scan for ANY satellites. I would have been better off starting it up and letting it sit on a table outside for awhile before going into the woods after the flight.

So if the unit has not properly acquired a GPS position, and you set off on a ride, it will then struggle to get one?

And the correct procedure is to turn the unit on outside and wait for it to get a signal before you start moving?

There's no totally correct procedure, but in general, that's correct. I always turn my GPS on when I'm unloading my bike from the car and I'm getting suited up. For most of my rides that are relatively close to home, this works well. When I leave from the house, it usually means I have to set the GPS outside to "warm up" while I'm getting ready, or I'm going to have to stand around and wait for it to get a signal. When I travel, I need to give it a little longer so I will fire it up at the hotel or campsite LONG before I intend to go for a ride or a run (hours or days).

Alternatively, turning it on inside a few minutes ahead of ride time and putting it on the sill of a south-facing window will give better signal than having it on in your basement or something.

It is a bit odd that it's *claiming* to have a signal and starts giving random speed readings indoors right away. Mine just gives no speed value until it has a pretty good signal, unless I take off riding with the wheel speed sensor spinning.

The above statements have not been evaluated by the Food and Drug Administration

If l turn the unit on indoors while l am getting ready, then l take the bike outside, l generally find that it has acquired a signal while it was indoors.

But if l then set off, the speed readings are totally inaccurate.
The speed will creep up to 20 mph or more, even though l am only doing 5mph. Eventually it settles down but it takes a couple of miles.

If l wait until l get outside before turning on the unit, this does not happen.

The problem is that after you let it start up inside, you just take off when you get outside, instead of giving it a minute to get a more accurate lock. You're going from a low accuracy environment to a better one, and there's a lag for the device to get a better lock. If you don't want the wacky readings, you have to give it a minute before you go anywhere.